1. Technical Field of the Invention
The present invention generally relates to the telecommunications field and, in particular, to an apparatus and method that compensates for a problematic time-varying DC offset by effectively eliminating a near-channel interfering amplitude modulated (AM) signal from a communications channel.
2. Description of Related Art
In the telecommunications field, one of the most significant design challenges involves the development of new direct conversion receivers that are capable of improving the demodulated quality of a signal. Traditional direct conversion receivers or homodyne receivers generally operate to demodulate an incoming signal by directly converting the incoming signal down to baseband, without the use of any intermediate frequencies, and outputting a desired signal. An example of the traditional direct conversion receiver is briefly discussed below with respect to FIG. 1.
Referring to FIG. 1 (PRIOR ART), there is illustrated a block diagram of a traditional direct conversion receiver 100. Basically, the traditional direct conversion receiver 100 includes an antenna 102 for receiving a signal from a transmitter 104. The received signal is filtered by a band pass filter (BPF) 106 designed to pass a desired frequency band such as the GSM (Global System for Mobile Communications) frequency band from the received signal. The filtered signal is amplified in a low noise amplifier (LNA) 108 and down-converted to a base band Inphase (I) component and a base band Quadrature (Q) component using mixers 114a and 114b, respectively, and a local oscillator (LO) 116. The local oscillator 116 outputs a frequency adapted to a carrier frequency of the received signal. The base band I and Q components are respectively filtered by first low pass filters (LPFs) 118a and 118b, converted to digital signals by analog-to-digital convertors (A/Ds) 120a and 120b, and then filtered by second low pass filters (LPFs) 122a and 122b to obtain a signal format that can be handled by a data recovery unit (DR) 124. The data recovery unit 124 operates to demodulate the received signal.
Traditional direct conversion receivers 100 have an efficient radio receiver architecture in terms of cost, size and current consumption. However, traditional direct conversion receivers 100 suffer from the well known DC offset problem that can be attributable to three different sources: (1) transistor mismatch in a signal path; (2) the local oscillator 116 outputting a signal that leaks and self-down converts to DC when passed through mixers 114a and 114b; and (3) a large near-channel amplitude modulated (AM) interfering signal leaking into the local oscillator 116 and self-downconverting to DC. Since, the resulting DC offset can be several decibels (dB) larger than the information signal, one should take care of the DC offset to be able to recover the transmitted data in the data recovery unit 124.
The DC offsets due to (1) and (2) can be assumed to be constant during one burst (i.e., a number of received symbols) and can be taken care of by adding an extra DC component to the signal model used while demodulating the transmitted data in the data recovery unit 124. This method is well known in the art. However, the DC offset due to (3) is time-varying because of the amplitude variations in the interfering signal and as such it is difficult to compensate for this particular DC offset. Two examples of how the traditional direct conversion receiver 100 can be adapted to compensate for such AM interfering signals are disclosed in WO 98/04050 and EP 0 806 841, and briefly described below with respect to FIG. 2.
Referring to FIG. 2 (PRIOR ART), there is illustrated a block diagram of a traditional direct conversion receiver 200 configured to compensate for AM interfering signals as described in WO 98/04050 and EP 0 806 841. The general idea disclosed in both of these documents is to add a third receiver 202 (in addition to the I and Q receivers described above) designed to compensate for the dominating AM interfering signal.
The traditional direct conversion receiver 200 excluding the third receiver 202 generally operates as the direct conversion receiver 100 described above wherein like numerals represent like parts throughout FIGS. 1 and 2. For purposes of the discussion related to the direct conversion receiver 200 of FIG. 2, the received signal can include a wanted signal yt and an unwanted near-channel interferer pt. Due to nonlinear effects in the low noise amplifier 108 and the mixer 114a it can be shown that the dominated output from the second low-pass filter 122a is a wanted I component It and a fraction of the squared envelope of the interfering signal a|pt|2. Likewise, the dominated output from the second low-pass filter 122b is a wanted Q component Qt and a fraction of the squared envelope of the interfering signal b|pt|2.
The third receiver 202 is designed to take into account the nonlinear effects within the low noise amplifier 108 and the mixers 114a and 114b which collectively operate to convert the interfering signal to a base band signal. The low noise amplifier 108 directs the received signal to a power detector (PD) 204 which functions to detect an envelope of the received signal. It should be noted that this detected envelope consists mainly of the envelope attributable to the near-channel AM interfering signal whenever the unwanted interferer pt is much larger than the wanted signal yt. The power detected signal is then converted into the digital domain by an analog-to-digital convertor (A/D) 206, filtered by a low pass filter (LPF) 208 and fed to a control unit (CU) 210 which multiplies the detected envelopes with estimated parameters â and {circumflex over (b)}. The estimated interfering signals â|pt|2 and {circumflex over (b)}|pt|2 of the distortion are respectively input to subtractors 212a and 212b and subtracted from the I and Q components to obtain “relatively clean” I and Q components. The “relatively clean” I and Q components are then input to the data recovery unit 124.
Even if the solution to the DC offset problem described in WO 98/04050 and EP 0 806 841 appears to be promising it still has disadvantages, in terms of cost and current, due to the need to implement a third receiver. Therefore, there is a need for an apparatus and method that can suppress the near-channel AM interferer in a cost and current efficient manner.